You can do this by creating a device value change trigger in homeseer. Then you select the device that acts as a trigger and specify the value is set to alert.
Device RFXCOM Exterior Garage Door value becomes equal to Alert
As an action you can add 2 actions:
Action 1.
run script action and enter a line to take the snapshot, i.e.
Run script: &hs.GetURLImage "http://user:password@192.168.1.123","/axis-cgi/jpg/image.cgi?camera=3",false,80,"/html/images/CamAlerts/GarageDoorLast.jpg"
Action 2 (optional if you want to send the taken picture by email)
Send Email To: mailto@domain.com
Subj: ALERT: Garage Door is OPEN
(Specify to attach the picture taken in the previous action)
Hope it helps
JD